Effect of Ozone on the Stability of Solution-Processed
Anthradithiophene-based Organic Field-Effect Transistors
Iyad Nasrallah†, Kulbinder K. Banger†, Yana Vaynzof†§, Marcia M. Payne‡, Patrick Too||, Jan Jongman||,
John E. Anthony‡, Henning Sirringhaus†*
†
Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA
||
Plastic Logic Ltd, 34 Cambridge Science Park, Cambridge CB4 0FX, United Kingdom
‡
We have investigated the degradation effects of ozone exposure on Organic Field-Effect Transistors (OFETs) based on 2,8Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES ADT) as the organic semiconducting channel layer, as well as on
thin films of this widely used, high mobility, small molecule semiconductor. Electrical I-V measurements showed a loss of
transistor characteristic behavior. We present 1H Nuclear Magnetic Resonance (NMR) spectroscopy results as well as X-ray
Photoemission Spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectroscopy measurements showing the oxidation of
the parent molecule, from which we suggest various possible reaction paths.
INTRODUCTION
The interest in organic transistors has seen consistent
growth over the past decade due to the new technological
applications which these devices can enable, but also the
scientific insight they provide about the charge transport
physics of organic semiconductors. OFETs have enabled
flexible display backplanes on plastic substrates and are also
being developed for a wide range of sensing, imaging and
logic functionalities embedded into flexible substrates.1
As these technologies are beginning to become available
commercially, one of the main concerns for organic
electronics is device stability. Many studies have been
conducted aiming to improve the stability and performance of
OFETs.2-4 Investigations are often carried out in ambient
environments or ones with high humidity or oxygen content.
Whilst these studies are vital, there have been very limited
reports characterizing the effect of trace gases, such as ozone,
which is known to readily degrade organic materials, in
particular molecules with sp and sp2 carbon centers. Chabinyc
et al.5 showed that ozone could be a major cause of changes in
the characteristics of thiophene-based conjugated polymers in
ambient air. Furthermore, not only is ozone present in the
atmosphere, but it is also a natural product of several
fabrication processes, especially those which require UV
illumination in an air environment.
This study has been conducted to characterize the effect of
sizeable concentrations of ozone on diF-TES ADT based
OFETs through various measurement techniques.
diF-TES ADT is a p-type small molecule (chemical
structure shown in Figure 1). It has proven to show good
crystallinity, and high mobilities of >1 cm2/Vs in solution-
processed OFETs.6 Hence, it is a widely used material for the
fabrication of OFETs.
EXPERIMENTAL SECTION
Top-gate, bottom-contact OFET structures were used
throughout the experiments (architecture shown in Figure 1).
All transistor measurements were performed on devices with
20µm channel length and 1mm channel width. Electrical
characterization was performed on 8 pristine transistors, as
well as 8 transistors for each ozone exposure time.
A 50nm Polyimide layer was spun on glass substrates from
a solution in 1-Methyl-2-pyrrolidone (NMP), and cured inside
a nitrogen glovebox for 1 hour at 160˚C then 3 hours at
300˚C. This provides a planarization layer, thus mimicking a
plastic substrate and improving the film forming properties of
diF-TES ADT. Photolithography and thermal evaporation
were used to pattern 2 nm/25 nm of Chromium/Gold as source
and drain electrodes. The electrodes were treated with
pentafluorobenzenethiol (PFBT) in Isopropyl alcohol (IPA)
solution in order to improve injection into the semiconductor.
This also helps to improve the crystal growth into the device
channel region.7
Figure 1. a) and b) show the change in the transfer characteristics due to low (0.15 ppm) and high (5-6.5 ppm) concentration ozone
exposures, respectively. c) shows the change in output characteristics due to low and high ozone exposures compared to a pristine OFET.
c) also shows the molecular structure of diF-TES ADT, as well as the device architecture of the top-gate bottom-contact transistors used.
In all the experiments, diF-TES ADT solutions were spun
inside a nitrogen glovebox using a constant spin-coating recipe
for consistency. Solutions were spun at 1000 rpm for 1 minute.
The films were then annealed at 100°C for 2 minutes to
evaporate excess solvent. Unless otherwise stated in the text,
the solvent used was Mesitylene (C9H12, Fluka, >99% purity).
In OFET characterization, the ozone exposure was carried
out on the diF-TES ADT thin film before spinning the
dielectric layer. It has been shown that the dielectric may act
as an encapsulation layer, hence protecting the
semiconductor.8
CYTOP was used as a gate dielectric, and was spun inside a
nitrogen glovebox to provide a thickness of approximately 500
nm. Finally, a 25 nm Aluminium gate electrode was thermally
evaporated. The OFETs were measured using a probe station
housed inside a nitrogen-filled glovebox.
Ozone exposure was carried out using a commercially
available ozone generator bought from Heaven Fresh (Model:
HF 10). The generator and the samples to be exposed were
placed in a diecast box with the ozone flow output placed
approximately 4 cm away from the samples. An ozone
concentration meter (Ecosensors Model: A-21ZX) was also
placed at a similar distance to the samples to measure the
concentration over time. The exposure was done for short
intervals at 0.15 ppm, as well as in half hour intervals at an
average ozone concentration of 5.0-6.5 ppm inside the diecast
box. The concentrations of ozone that are present in ambient
air are typically 0.08 ppm. Smog conditions in cities provide
ozone concentrations of typically 0.1-1 ppm. The high
exposure concentrations of 5-6.5 ppm were used to accelerate
the degradation effects and saturate all chemical reaction paths
within a practically observable time scale of a few hours.
Absorption spectra were performed on 30 nm films of diFTES ADT spun on Spectrosil substrates. The measurements
were taken in a vacuum in the order of 10 -6 mbar, using a
Varian Cary 6000i Spectrophotometer with a spectral range
from 175 nm to 1800 nm. The samples were exposed to 5-6.5
ppm of ozone.
For
X-Ray
Photoemission
Spectroscopy
(XPS)
measurements, 30 nm layers of diF-TES ADT were spun on
silicon substrates coated with a 50 nm layer of thermally
evaporated gold to assist with charge compensation. The
measurements were done using a Thermo Scientific Escalab
250Xi XPS/UPS system. Both low (0.15ppm) and high (5-6.5
ppm) concentration ozone exposure was carried out.
Thin films of diF-TES ADT with 100 nm thickness were
spun on glass substrates for Proton (1H) Nuclear Magnetic
Resonance (NMR) measurements. Toluene (C7H8, ROMIL,
Hi-Dry anhydrous) was used as a solvent for spinning. Pristine
films were dissolved off using anhydrous deuterated
Chloroform (CDCl3, Sigma-Aldrich, 99.8% purity anhydrous)
in air, immediately after spinning. The remaining thin films
were exposed to 5-6.5 ppm of ozone for a total of 6 hours.
These were then dissolved off in a similar manner to the
pristine films.
For Fourier Transform Infrared (FTIR) spectroscopy 100
nm thick diF-TES ADT films were spun on Potassium
Bromide (KBr) pellets. 5-6.5 ppm ozone exposure was carried
out for 7 hours. The IR spectrum of several pristine and ozone
exposed films were taken to confirm the change induced in the
spectrum due to the exposure.
RESULTS & DISCUSSION
Figure 1a shows that upon ozone exposure at 0.15 ppm for
10 to 20 minutes, a general reduction in p-type mobility from
an average of 0.7 cm2/Vs in the pristine state to 0.1 cm2/Vs is
seen, with the ON current reducing by an order of magnitude.
We also see evidence for p-doping in this initial phase of
exposure, which manifests itself as a more prominent shoulder
in the positive gate voltage range. However, the dominant
effect even at this low concentration is the dramatic drop in
ON current, the significant shift of the turn-on voltage to more
positive gate voltages, and the appearance of significant
hysteresis. After 80 minutes of ozone exposure, we see that the
transistor performance is degraded further, with larger
reduction in mobility and even more apparent hysteresis. The
sign of the hysteresis is such that the current is higher on the
forward scan from OFF-to-ON conditions than on the reverse
scan. In Figure 1b we see that when we use higher ozone
concentration a very similar degradation behavior is observed
and a similar non-working state of the device is already
reached after half an hour of exposure.
This behavior can be explained by the ozone exposure
degrading the molecule, and creating species that can trap
electrons. During the positive gate voltage part of the forward
transfer characteristics electrons are injected from the sourcedrain electrodes, but then quickly form negatively charged trap
species inside the semiconducting layer without leading to a
measurable mobile electron current. This negative space
charge then facilitates the formation of hole accumulation
already at positive gate voltages giving rise to a positive turnon-voltage. While operating the device in the ON state the
injected holes then recombine with these negative trapped
electrons leading to a negative shift of turn-on voltage during
the ON-part of the transfer measurement and giving rise to the
observed hysteresis. In this regime the onset voltage shifts to
more positive values as the transfer characteristic is started at
more positive voltages (shown for 80 minutes in Figure 1a by
the dashed blue line, and 0.5 hours in Figure 1b by red lines of
varied style). The dramatic decrease in the ON current upon
ozone is ascribed to the fact that any molecular species formed
by a chemical oxidation reaction with ozone are likely to have
a larger band gap and hinder efficient hole transport. After
prolonged exposure more and more of the film is oxidized and
after 1-1.5 hours the device is no longer operational. A similar
degradation also appears in the output characteristics upon
ozone exposure.
Figure 2. a) and b) show absorption spectra of diF-TES ADT with increased ozone exposure (at 5-6.5 ppm), with a) showing the
change in the vibronic progression. c) shows the change in color of the thin film due to ozone exposure. d) shows the loss of
crystallinity in the film with increased ozone exposure.
The ideal output characteristic shape is lost after 80 minutes
respectively. Comparing the two spectra, we can see the
of low concentration ozone (Figure 1c). The current ceases to
emergence of two new peaks at 1750 and 3500 wavenumbers
saturate whilst the overall current magnitude decreases
after ozone exposure. These are in the correct regions
severely with increased exposure. Hysteresis also becomes
assignable to the C=O bond stretch and the O-H bond stretch,
evident. The sign of the observed hysteresis in the output
respectively.9
characteristics may be explained by assuming that under
conditions where |Vd|>|Vg-VT|, i.e. in saturation, negative
electrons are injected and trapped near the drain electrode in
ozone generated species leading to a dynamic shift of the turnon voltage towards positive gate voltage and a temporary rise
in current on the reverse scans in the output characteristics.
Figure 2b shows that the reduction of absorption strength
associated with the higher lying * transition around at 330340 nm indicates that the conjugation length of the
chromophore is being shortened due to the exposure. This is
accompanied by new absorption peaks at shorter wavelengths
in the ultraviolet (UV) region (below 280 nm). This shows that
ozone leads to the oxidation of the molecules and creates
molecular species with a higher energy gap. Figure 2c shows
the discoloring of the film after 7 hours of exposure at 5-6.5
ppm. Furthermore, using an optical microscope with crosspolarizers, it was possible to observe a loss of the typical
polycrystalline morphology of the film; after 1.5 hrs of
Figure 3 FTIR results confirm the formation of C=O and O-H
exposure the films appeared near amorphous (Figure 2d).
bonds due to ozone exposure.
The FTIR spectra for pristine and ozone treated films are
shown in Figure 3. The fingerprint features at around 2300 and
2900 wavenumbers correspond to CO2 and C-H bonds
Figure 4. The results of XPS on pristine and ozone exposed thin films of diF-TES ADT. The elements shown are oxygen (O1s), silicon
(Si2p), fluorine (F1s), sulphur (S2p) and carbon (C1s). The 20 minute and 80 minute exposures are performed at low ozone concentrations.
The rise in the overall background signal below 1000
wavenumbers after ozone exposure can be attributed to the
formation of O-O bonds belonging to endoperoxide groups.10
The breadth of these newly formed peaks could collectively
raise the background signal when superimposed on the already
existing signals.
XPS (Figure 4) was used to characterize the change in the
chemical composition of the films with increased exposure.
The scans at 20 minutes and 80 minutes were performed at
low ozone concentration exposure (0.15 ppm). The trends seen
in the data are initiated even at short exposure times, and low
concentrations. Whilst the oxygen (O1s) peak for the pristine
film (shown in black) shows no features, a signal at
approximately 532.5 eV begins to emerge with increased
exposure time. The emerging O1s peak is very broad, which
indicates that there are several oxidized moieties contributing
to this signal. For the remainder of the constituents (silicon
(Si2p), fluorine (F1s), sulphur (S2p) and carbon (C1s)) we
observe a reduction in signal and a broadening of the
respective peaks. This is clearly seen in the case of S2p, for
example, for which the initial characteristic doublet peak
shape of sulphur becomes significantly broadened and
decreases in intensity.
Due to the high electronegativity of oxygen, oxidized
moieties are seen in the XPS spectra for the ozone damaged
films at higher binding energies than their respective pristine
reference peaks. This is clearly seen in the cases of S2p, where
a new broad peak begins to appear at a higher binding energy
(168 eV).
In the case of C1s, the peak at around 287 eV
corresponding to the carbon-fluorine bond broadens whilst
maintaining a significant intensity. This is also the correct
region for carbon-oxygen bonds (single and double bonds),
hence the broadening is due to new contributing signals from
oxidized carbon.
A new C1s peak at 288.8 eV begins to emerge with
increased exposure time, even at low ozone concentrations.
This peak is 4.3 eV away from the main carbon peak located
at 284.5 eV. Cross-referencing with XPS libraries and
previous publications, this peak can be assigned to carboxylic
acid groups.11-14
To obtain further insight regarding the change in molecular
structure of diF-TES ADT upon ozone treatment, NMR
analysis was conducted. The 1H NMR of pristine diF-TES
ADT was also recorded to serve as a reference, (Supporting
Information–S1). The spectrum of the pristine material shows
chemical environments which can be fully assigned, and agree
with spectra reported previously in literature.15 The full 1H
NMR for the ozone treated diF-TES ADT is also shown in
Supporting Information-S1. Figure 5a clearly shows that the
chemical environments located at 1.2 ppm and 0.9 ppm in the
pristine film (black line in Figure 5a) are no longer evident
after ozone exposure (red line).
The NMR data on the ozone treated film also suggests a
change of proton environment just below 9 ppm (Figure 5b,
and Supporting Information–S2), corresponding to both ends
of the anthracene core (C-C=C-H region) in the vicinity of the
thiophene rings.
The complexity of the reaction of ozone with organic
compounds has been previously conveyed in literature.16 The
strong reactivity of ozone allows for many possible oxidation
pathways to take place, and hence several products to be
formed. The FTIR data has revealed the formation of C=O
bonds, which attach to the chromophore forming quinones (or
semiquinones).17
This supports the change in proton
environment along the chromophore seen in NMR (Figure 5b,
Supporting Information–S2), and the oxidation of sulphur seen
in XPS. Quinones are the strongest contenders to explain the
hysteresis seen in the OFET electrical characteristics as they
have high electron affinities.
Furthermore, the disappearance of the NMR peaks at 1.2
ppm and 0.9 ppm suggests that the chemical environment
experienced by the alkyl side chains is now dissimilar to when
attached to the anthracene core for pristine diF-TES ADT. It
has previously been shown by Fudickar et al.19 that the triple
bond in the alkyne side chains of acenes act to stabilize the
molecule by preventing the molecule from decomposing as a
result of oxidation. Endoperoxides (also shown in our FTIR
data) on the central ring are formed as a result, with the
reaction being reversible to the pristine molecule by
thermolysis. The method of oxidation described in the abovementioned study involves photooxidation as a result of
reacting with singlet oxygen. The reaction with ozone in our
investigation is expected to be much more damaging, resulting
in permanent degradation of the molecule.
CONCLUSIONS
Figure 5. 1H NMR results showing the change of chemical
environment at a) the side chains (0.9ppm to 1.2ppm), and b) the
chromophore.
The new peak in the C1s XPS data at 288.8 eV is most
likely derived from the reaction occurring to the side chains.
Criegee and Lederer18 showed that the reaction of ozone with
alkynes in the presence of water produces carboxylic acid
derivatives,16 as shown in Figure 6.
Figure 6. A highlighted reaction path in the Criegee mechanism
which best describes the reaction taking place at the side chains.
This is further corroborated by the appearance of an O-H
bond peak in the FTIR data, a constituent of carboxylic acid
groups. The XPS data agrees very closely with the path
highlighted in Figure 6, due to the humidity present in air
during the exposure.
We have shown that ozone exposure has a detrimental nonreversible effect on the performance of OFETs based on diFTES ADT as the semiconducting layer. OFET devices show a
marked decline from their pristine working state to a
completely redundant form after ozone exposure in air. We
have examined the oxidized films in comparison with their
pristine analogues using UV/VIS, NMR, XPS and FTIR.
Our study points us to suggest that diF-TES ADT
decomposes through several oxidation pathways. We are able
to link our experimental results to established reactions in
literature, such as the formation of quinones, and the
formation of a carboxylic acid compounds. The quinones
introduce strong hysteresis in the OFET electrical
characteristics due to their high electron affinities.
Furthermore, the destruction of the molecule is the reason for
the loss of ideal charge transport. Our primary objective in this
work has not been to conclusively identify the final reaction
product (or products) due to the exposure of diF-TES ADT to
ozone. This is beyond the scope of our investigation and might
be addressed by future High Performance Liquid
Chromatography / Mass Spectrometry (HPLC/MS). Our aim
here has been to understand the relationship between device
performance and ozone exposure.
These reactions occur throughout the bulk of the film and
are responsible for the dramatic degradation of the device
characteristics upon ozone exposure. We would like to
emphasize that the observed reactivity of diF-TES ADT
towards ozone is not a consequence of the relatively high
exposure concentration used here. We have also observed
similar degradation effects at lower concentration albeit over
longer timescales. Our results have important implications for
the fabrication and operation of organic FETs based on diFTES ADT. During device fabrication care needs to be taken to
minimize the exposure of the films to ozone, particularly
during the critical stages of manufacture where the
semiconductor film is not yet protected by the gate dielectric
layer. Ozone is often present in cleanroom environments
where UV cleaning and other UV operated equipment is used.
We anticipate that depending on the level of air encapsulation
provided, ozone may also be an important contributing factor
to long term device degradation during storage and operation.
ASSOSCIATED CONTENT
Supporting Information S1 – Pristine and Ozone treated diF-TES
ADT NMR Spectra
Supporting Information S2 - Change of Proton Environment
around the Anthracene Core
This material is available free of charge via the Internet at
http://pubs.acs.org
AUTHOR INFORMATION
Corresponding Author
*E-mail: hs220@cam.ac.uk
Present Addresses
Dr. Yana Vaynzof has relocated to the following address:
§Centre for Advanced Materials, Universität Heidelberg, Im
Neuenheimer Feld 227, 69120 Heidelberg, Germany
ACKNOWLEDGEMENTS
The authors would like to thank Plastic Logic Ltd. for initiating
and funding this project.
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